The invention concerns valves for the control of pressure fluid and is especially concerned with the sealing arrangements by which interport and other unwanted leakages are prevented. Although the invention has particular application to valves of the construction characterised by a plunger or shuttle-like mobile member that moves within a valve body to control the communication between sets of ports in the latter, it will become apparent that the invention is of more general utility, being applicable to any valve construction in which a mobile member controls communication between ports in a valve body and unwanted interport and other leakages of the controlled pressure fluids are restrained by a flexible or resilient sealing element that is of strip-like form, double-ended or formed into a loop, and housed in a recess in the mobile member or valve body in such manner as to be distorted into sealing engagement with appropriate surfaces of the recess, mobile member and valve body by the fluid pressure differential tending to cause an unwanted leakage past the sealing element. Accordingly, whereas the invention is applicable to a wide variety of valve constructions, for instance to sliding plate valves and poppet valves, for convenience it will be described and discussed with reference to its application to valves having a plunger or shuttle-like mobile member. Valves of this construction are of two types, those having the sealing elements fitted to the mobile member and those having the sealing elements fitted to the valve body.
When the sealing elements are fitted to the valve body the mobile member has peripheral grooves and is spool-like in shape so that the valve is generally designated a "spool valve", whereas when the sealing elements are fitted to the mobile member the valve is generally described as a "piston valve". In general, the piston valve has the more simple construction, whereas the spool valve, although of more complicated construction, usually has the advantage of larger fluid flow capacity for a given physical valve size, and a better seal life expectancy because the seals are fixed with respect to the ports and the mobile member can be shaped to give smooth seal engagement and release.
The seals commonly used in valves of this class are made from an elastomeric material such as, for instance, neoprene or nitrile rubber. The seal design and material are so chosen that under pressure from the fluid controlled by the valve a seal will undergo deformation to provide pressure-tight sealing against the surfaces with which it is engaged. Considerable care is necessary in the choice of seal design and material for a particular duty in order to avoid both leakage arising from inadequate seal deformation under working pressures and also excessive deformation likely to cause damage to the sealing surfaces engaged by a seal.
In spool valve designs, the shape of the mobile member includes chamfered or radiused lands for the isolation and interconnection of the various ports in the valve body as the spool is shifted. Seals such as O-rings are located in the body between the ports therein, in grooves that are usually defined by spacers of suitable configuration. Such spacers have radial holes for the passage of the controlled pressure fluid and if the seals are of the O-ring type, the ends of the spacers are usually shaped in such a way as to provide restraint to prevent inward collapse of the seal elements into the spool grooves during movement of the spool. In some cases, however, especially when the seals are of a composite form that is intrinsically non-collapsible, the spacers do not require to have specially shaped ends to restrain the seals. In general, however, seal restraint against collapse is required when the valve is to control fluid under relatively high pressure.
In piston valve designs the seals move relatively to the ports and traverse some at least of the latter. For this reason it is most usual to arrange the porting in a bush that fits into the valve body so that the ports subject to seal traverse may take the form of a large number of small diameter holes into which a seal passing thereover has little opportunity to have its surface extruded and so be subject to wear and/or damage. However, such a bush, with its large number of small diameter holes is relatively costly to manufacture, and the many small diameter holes break up the flow of pressure fluid through the port and cause loss in efficiency. Moreover, when a valve of this construction is used to control fluid at high pressures, some deformation of seal surface into the holes during movement of the piston is almost inevitable, with consequent wear of the seals.
A less common form of piston valve construction involves ports in the form of narrow annular slots in a bush or bush assembly fitted to the valve body. This design can give a better fluid flow, for a given total port size, than the described port design involving many small holes but there remains some tendency for the seal surface to extrude into the annular slots as the piston moves, especially when the valve has to control a fluid at relatively high pressure. On the other hand, this design has an additional advantage in that the valve can be constructed to accomplish the required changeover of port interconnection with a very short travel of the mobile member, because the required travel is determined by the port width that can be narrow in the case of annular slot-like ports.